SELECT语句中的Linq静态方法

Question

I'm working with EF and have some queries. Here is my code

IEnumerable<Customer> customers = from c in context.Customers 
    select new Customer
    {
        ID = c.ID,
        Name = c.Name,
        LastName = c.LastName,
        DepID = c.DepID,
        Editable = SomeStruct.Check(c.DepID)
    }

public struct SomeStruct
{
    public static bool Check(int depID)
    {
        //Here I have some logic
    }
}

It works fine. However, if I declare SomeStruct as class it will fail.

My questions are:

1. Why does it happens ?
2. Does using of static function forces the query to execute ?

Answer 1

In your code method SomeStruct.Check(c.DepID) should be transformed to SQL query. This describes behaviour with class/structs and so on. It is due to different work of Entity Framework with such methods in class and structure. But you can do this check on client:

IEnumerable<Customer> customers = from c in context.Customers 
    select new
    {
        ID = c.ID,
        Name = c.Name,
        LastName = c.LastName,
        DepID = c.DepID
    }
    .AsEnumerable()
    .Select(d=>new Customer
    {
        ID = c.ID,
        Name = c.Name,
        LastName = c.LastName,
        DepID = c.DepID,
        Editable = SomeStruct.Check(c.DepID)
    });

Or you can set Editable property as readonly property:

public class Customer{
    public int ID{get;set;}
    public string Name{get;set;}
    public string LastName {get;set;}
    public Guid DepID{get;set;}
    public bool Editable{get{return SomeStruct.Check(DepID);}}
}

Answer 2

It is easily reproducible that your code can not work with linq to entities. Anything that can not be translated into sql will throw a runtime exception. In you case:

NotSupportedException: LINQ to Entities does not recognize the method 'Boolean Check(Int32)' method, and this method cannot be translated into a store expression.

If you got it running at some point, that can not have been related to struct or class differences, see What's the difference between a static struct method and a static class method?

You probably changed something else at the same time, like adding ToList() before the select .

Answer 3

Supposing it's a runtime issue, in many cases you are describing, this is a reference issue. EF's linq provider supports calling a static method declared in a class (static or not).

1. I recommend that you to change your struct to a class AND make it static to help you investigate the issue.

2. No, nothing will be executed before you calling a first iteration or customers.

Hope this helps